Repeaters are deployed to increase the coverage area of a wireless communication system. A repeater is comprised of two assemblies: Donor and Service. The Donor assembly receives data from Donor base station, amplifies, filters the received signal and transmits the amplified and filtered signal from radio unit. Communication with the base station is done via the Donor antenna. The Service assembly receives data from service radio unit, amplifies, filters the received signal and transmits the amplified and filtered signal from the base station. Communication with radio unit is done via the Service antenna.
Repeaters use Finite Impulse Response (FIR) filters as part of signal processing. In general, FIR filter synthesis is constrained by pass band ripple, stop band rejection, roll-off, group delay (GD) and group delay distortion (GDD). The distortion the filter imposes on the signal causes an increase in the signal's error vector magnitude (EVM). Pass band ripple both in the amplitude and in the phase domain contribute to the added EVM from the digital filter and the phase ripple is closely related to the GDD. The latency or GD is closely related to the filter length which is also related to the cost of implementation in terms of resource of the programmable circuit. One wishes to reduce both cost and latency of course. Improving the filter on some parameters, results in degradation of some other parameters. For example, targeting for better group delay limits the improvement on other parameters like pass band ripple or stop band rejection, etc. The stop band rejection on the other hand must be sufficient for at least two reasons. First, the regulatory body limits the out of band gain and thereby the “gain” of the repeater is limited. Second, a larger gain in an adjacent frequency band means that some signal in the band of interest can sneak through the adjacent bands filter and interfere at the common output thereby causing EVM. Thus a filter is also characterized by the allowed gain difference “Difference Gain” a filter can have without destroying the traffic passing mainly through an adjacent sub-band. The following Table 1 illustrates the above mentioned trade-off between signal performance parameters.
TABLE 1Filter parameter specificationsGroupDelayFilterBWDifference#(nS)Length(MHz)Gain (dB)Gain (dB)Ripple/EVM19.0350.2100250.228.0310.2100200.336.0270.290200.3
With current filter techniques available, it is not possible to generate a filter with all the parameters at their best. FIG. 1 shows FIR Filter synthesis constraints trade-off. Repeater requirements vary depending on many factors such as Radio Access Technology (RAT) supported, Bandwidth available, Channel spacing, Repeater gain, Repeater Delay. Different repeater requirements imply different filter constraints. Repeater requirements limit improvement of some filter parameters while targeting other filter parameters. And thus, a fixed FIR filter structure does not accommodate all of the different repeater requirements.
Trade-offs in filter structure are shown in FIGS. 2 and 3 with regard to the following examples: a) for the same group delay of 2.8 μs, higher rejection can be achieved by sacrificing the outer most band or by allowing higher ripple in the pass-band; and b) frequency plots in FIGS. 2A and 2B show a 5 MHz filter for a same group delay that has lower rejection with a relatively flat pass band than the one shown in FIGS. 2C and 2D, which has a higher rejection but with a greater ripple in the pass band. It is well known that given a higher allowance in group delay, better rejection, flat pass band etc., can be obtained. A 5 MHz filter with a higher group delay (4 μs) is shown in FIGS. 3A and 3B.